How about a card like this
(two dark domains) 4 
(cost)Two adjacent target undead creatures are combined together
hit points, and defensive points get added together
special features are either melded(you can only use 1 a turn so you'd have to choose), or you'd pick the one you want and the other one gets negated at the time of casting, or they could both be negated at the time of casting (i am not sure as to what the game could handle)
if you meld a flyer with a walker, the new unit would have hover
This could potentially create the games strongest creature...or just bulk up a couple of wimpies